Device for scanning an object with a light beam

ABSTRACT

Disclosed in this specification is a scanning device which scans an object having a flat reflection surface and an inclined reflection surface with an inclination relative to the flat reflection surface such as, for example, a mask and a wafer to be used in manufacturing IC, LSI, etc., with light beam, and detects only the reflected light from the inclined reflection surface with a light detector. In order to make it possible to detect only the reflected light from the inclined reflection surface with the light detector, a telecentric lens is used as the scanning lens in this scanning device, and the original point of deflection of the above-mentioned light beam coincides with the center of the pupil of this telecentric lens. In addition, a filter is disposed on the pupil surface to intercept light from the flat reflection surface.

BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention relates to a scanning device, and, more particularly, itis concerned with a scanning device which scans with a light beam anobject having a flat reflection surface and an inclined reflectionsurface with an inclination relative to the flat reflection surface, anddetects with a light detector only the reflected light from the inclinedreflection surface.

b. Description of Prior Arts

There are many kinds of objects having a flat reflection surface and aninclined reflection surface. In the present specification, explanationwill be made with reference to an alignment mark on a mask and a waferfor use in fabrication of IC and LSI, as an example.

In general, alignment of the mask and wafer has so far been done bymoving in parallel any one of the mask and the wafer relative to theother until the alignment marks on both of them come to a predeterminedpositional relationship. In order to carry out the appropriatealignment, the alignment marks provided on both mask and wafer must beobserved.

For the method of observing this alignment mark, there have been knownthe so-called "bright sight observation", in which reflected light fromthe flat reflection surface is observed, and the so-called "dark sightobservation", in which the reflected light from the inclined reflectionsurface is observed. The scanning device of the present invention isconcerned with the latter method, i.e., dark sight observation, aboutwhich more detailed explanations will be made hereinbelow.

A device for observing an object having both a flat surface and aninclined surface, such as the alignment mark of a wafer for IC patternprinting in a dark sight, has already been proposed in U.S. applicationSer. No. 672,022 now U.S. Pat. No. 4,062,623, filed Mar. 30, 1976 andhaving the same assignee as that of the present application. While thedevice in this earlier application will be explained with detail inreference to a drawing illustration at a later paragraph, it can beoutlined as follows. An image of a light source for illuminating thesurface of an object and which is smaller in size than the pupil of atelecentric lens is formed on the pupil surface of the lens, then theentire region of the object to be observed is illuminated, and theregular reflected light from the flat surface within the region to beobserved is intercepted by a light intercepting plate of a sizesubstantially corresponding to the image of the abovementioned lightsource disposed on the pupil surface of the telecentric lens or theimage surface of the pupil, whereby an object image to be formed by anon-regular reflected light from the inclined surface within theabovementioned region to be observed and passing through the lightintercepting plate (i.e. dark sight image) is scanned.

In this specification, the term "telecentric lens" refers to a lenshaving such a property that the principal light ray becomes parallelwith the optical axis, when the light source is disposed at anintersection of the optical axis of the lens and its pupil surface. Bythe term "flat surface", it is meant such surface that intersectsorthogonally with the optical axis of the telecentric lens. Also, by theterm "inclined surface" is meant such surface that does not intersectorthogonally with the optical axis of the telecentric lens.

As is understandable from the above explanations, the invention asdisclosed in the earlier application illuminates the entire region ofthe object to be observed, and sequentially detects light from a part ofthe region, on account of which only a very small portion of the lightamount for illuminating the object is used at the time of the lightdetection. Thus, it cannot be said that the illuminating light isutilized in an effective manner.

The present invention relates to improvement in the prior invention inthe abovementioned earlier application, the point of improvement ofwhich resides in utilizing this illuminating light to the object aseffectively as possible. For this purpose, the device concerned adoptsthe so-called "flying spot light scanning system", in which the objectsurface is sequentially scanned with a relatively thin scanning lightbeam, and the light reflected from this object surface is detected.

It should, however, be noted that the present invention is not a simpleapplication of the flying spot light scanning system to the device ofthe abovementioned earlier U.S. application Ser. No. 672,022 now U.S.Pat. No. 4,062,623, but solves a problem which is liable to occur whenthis flying spot light scanning system is adopted to the device of theprior invention. This problem is derived from the fact that, when thescanning beam is projected into the telecentric lens, the position ofthe scanning beam crossing the pupil plane of this telecentric lensvaries due to deflection or swerving of the scanning beam, on account ofwhich the position of the scanning beam crossing the abovementionedpupil surface is varied, when the reflected light from the flat surfacetravels backward. As the result of this, there takes place suchpossibility that the reflected light from the flat surface (regularreflected light) becomes unable to be removed due to presence of thelight intercepting member disposed on the pupil surface of thetelecentric lens or the image plane of the pupil.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a scanningtype light detecting device which has solved the abovementioned problem,and which is capable of utilizing the illuminating light in a mosteffective manner.

The above purpose of the present invention can be attained by makingconstant the position where the scanning beam crosses the pupil plane ofthe telecentric lens, i.e., by causing the original point of deflectionof the scanning beam to coincide with the pupil surface of thetelecentric lens.

The foregoing object and other objects of the present invention willbecome more apparent and understandable from the following detaileddescription thereof, when read in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF DRAWING

In the drawing:

FIGS. 1 and 2 are respectively schematic diagrams of the light scanningdevice of a dark sight type as disclosed in an earlier application;

FIG. 3 is also a schematic diagram showing an optical layout of a firstembodiment of the present invention;

FIG. 4 is a schematic diagram of a main part of the optical system shownin FIG. 3 for the purpose of explanation;

FIG. 5 is a schematic diagram showing an optical layout of a secondembodiment of the present invention;

FIG. 6 shows a sight of a microscope in accordance with the secondembodiment of the present invention shown in FIG. 5;

FIG. 7 is a schematic diagram showing an optical layout of a thirdembodiment of the present invention; and

FIG. 8 is a schematic diagram showing an optical layout of a fourthembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, which shows the optical layout of thescanning light detecting device of a dark sight type as proposed in theafore-mentioned earlier U.S. patent application Ser. No. 672,022 nowU.S. Pat. No. 4,062,623, a reference numeral 1 designates an object tobe scanned. This object 1 to be scanned has a flat surface 2 and asurface 3 having an inclination relative to this flat surface. Areference numeral 4 indicates a telecentric lens, and another referencenumeral 5 designates an aperture disposed on the pupil surface of thetelecentric lens 4. A reference numeral 6 denotes an illuminatingoptical system to form a light source image of a size smaller than thepupil diameter of the telecentric lens on the pupil surface thereofthrough a half mirror 7. A numeral 8 refers to a relay lens, and 9denotes a scanner having an open slit and disposed on the image surfaceof the object 1. A reference numeral 10 designates an image forming lensfor the pupil 5, and 11 indicates a light intercepting plate having aring-shaped opening disposed at an image position of the pupil of thetelecentric lens 4. The light intercepting portion corresponds to thesize of the light source image formed on the pupil of the telecentriclens 4. A reference numeral 12 designates a light converging lens, and13 refers to a light detector. As stated in the preceding, since thetelecentric lens 4 has such a property that the principal light ray ofthe light beam becomes parallel with the optical axis of the lens, whenthe light source is disposed at the intersection of the optical axis andthe pupil plane of the lens, the reflected light A (see FIG. 2) fromthis flat surface 2 is again converged on the position of the lightsource image, when the flat surface 2 is made perpendicular to theoptical axis. However, as the light projected onto the inclined surface3 is diverted in its travelling direction, it does not return to theposition of the light source image. Accordingly, the light from thelight intercepting plate 11 consists solely of the reflected light Bfrom the inclined surface 3 with the result that the light to bedetected by the light detector 13 is only that from the inclined surface3 of the object 1. This detection system will hereinafter be referred toas "dark sight detection system".

The light scanning method as disclosed in the earlier U.S. applicationSer. No. 672,022 now U.S. Pat. No. 4,062,623 forms an object image byilluminating the entire surface of the object 1, and the light from apart of the image is taken out sequentially by the scanner 9 to be ledto the light detector. On account of this, the detection efficiency ofthe illuminating light is extremely low, so that a problem arises inrespect of light amount.

The present invention, in place of scanning the image on the imagesurface, adopts a system of scanning an object with a light from theilluminating system by forming it on the object surface in the shape ofa spot or a slit. According to this system, since the light which hasheretofore been used in irradiating the overall surface of the objectcan be concentrated onto the position where a signal is to be taken out,remarkable increase in the detected light amount can be attained.Reduction in size of the spot can be realized by attaching a collimatorlens of a long focal length to the illuminating light source to therebyconverge light. In an other way, a small-sized spot light can be readilyobtained by the use of a laser beam. For illuminating the object surfacewith a moving light beam, it is necessary that the scanning beam beprojected into the telecentric object lens. In this case, however, thescanning beam moves even on the pupil of this object lens. At this time,since the position of the reflected light A to be intercepted tends tomove, along with the scanning operation on the object surface, even at aplace of the light intercepting plate 11 which is the conjugativeposition of the pupil, separation of the reflected light B for detectionbecomes difficult.

The present invention has succeeded in removing such defect to takeplace with the direct scanning of the light beam on the object surface,and has, as its aim, the original point of deflection of the scanningbeam at the intersection of the pupil plane and the optical axis of theobjective lens, thereby causing the position of the beam on the pupilsurface to be substantially unmovable, no matter how the scanning beamscans the object surface. This original point for deflection has a closerelationship with the scanning system.

As the scanning means, there has been known a rotating parallel flatplate. A transmitting type rotary polygonal mirror and vibrating typeparallel flat plate of a photo-electric microscope are the examples. Inother words, this scanning means utilizes the parallel flat plate forlight transmission, and, by causing this parallel flat plate to inclinewith respect to the optical axis, the light beam is shifted sidewise. Inthis case, the property of the parallel flat plate is such that itretains the parallelism, even when the incident light and the reflectedlight have shifted sidewise. Accordingly, it can be understood that thelight beam after its passage through the parallel flat plate is merelyshifted sidewise in the optical axis direction with the angle thereofbeing unchanged. In this case, the aberration to occur due to theinclination is almost negligible in view of the fact that the F-numberis ordinarily large and the picture angle is small. In order that thepositional discrepancy of the scanning beam on the pupil of thetelecentric lens may be made substantially zero at the time of thescanning operation, in other words, in order to position the originalpoint of deflection on the pupil of the telecentric lens, it may besufficient that a relay lens is interposed between the parallel flatplate for scanning and the telecentric object lens, and the focal planeof the relay lens is coincided with the pupil surface of the telecentricobject lens. When the reference light path of the principal light ray ofthe beam to be scanned (i.e., center of the beam) is taken as theoptical axis (that is, when the principal light ray is projectedperpendicularly onto the parallel flat plate), the principal light rayof the beam remains to be parallel with it being shifted sidewiserelative to the optical axis, as already explained in the foregoing,even if the parallel flat plate inclines for the scanning operation.This parallel principal light ray group to occur with the scanningoperation passes through the focal point of the relay lens withoutfailure after it has passed through the relay lens. In other words, thebeam does not move on the pupil surface, even if the scanning operationis carried out.

For the scanning means, there has also been known a rotary reflectionmirror. Examples of such scanning means are a galvano mirror, areflective type polygonal mirror, and the like. In this case, the angleof the principal light ray of the beam after it is emitted from thescanning means is in no way parallel as in the case of the transmissiontype, so that an entirely different method should be used for the beamnot to be deflected on the pupil surface. For this purpose, there may beutilized such a principle that the light beam which is being projectedonto the scanning means does not almost change its reflecting point onthe scanning means. That is to say, the reflecting point on the scanningmeans and the center of the pupil plane of the telecentric lens can bebrought to a mutually conjugative relationship through an image formingsystem. For this purpose, the relay lens and a field lens shouldappropriately be located between the scanning means and the objectivelens. The reflecting point of the light beam on the polygonal mirror andreflective mirror may move more or less due to rotation of such mirror,although the deflection is so small that it does not raise anyoperational problem or inconvenience. Accordingly, the beam position onthe pupil surface may be presumed to be almost unmovable along with thescanning operation.

In the following, the present invention will be explained in particularreference to the optical layout as shown in the accompanying drawing.

Referring to FIG. 3, which indicates the first embodiment of the opticallayout according to the present invention, a reference numeral 20designates an opening in the form of a slit or spot which is disposed inthe light beam from the light source, and which is in a conjugativerelationship with an object 1. A reference numeral 21 designates ascanning system of a type which causes the light from the opening 20 tomove parallelly such as, for example, a transmitting type scanningsystem consisting of a rotatable glass block as already mentioned. Arelay lens 22 is so disposed that its rear side focal plane may coincidewith the pupil 5 which constitutes the front side focal plane as well asthe pupil surface of the telecentric object lens 4. FIG. 4 brieflyindicates the function of this lens. In this illustration, the opening20 is coincided with the front side focal plane of the relay lens 22 forsimplicity and clarity in explanations. The light beams 23 after theyhave passed through the scanning device 21 are merely shifted sidewiserelative to each other on the principle that the incidence angle isequal to the projection angle, which is the property of the parallelplanes. Accordingly, each beam corresponds to such a situation as if itwere emitted from the focal plane of the relay lens 22, and it becomes aparallel light after its passage through the relay lens 22. Here, sincethe principal light ray which is the center of each beam is parallelwith the optical axis after its emission from the scanning system 21, itpasses through the center of the pupil 5 which is the rear side focalplane of the relay lens 22 as well as the front side focal plane of theobjective lens 4, after its passage through the relay lens 22. The angleof the light passage depends upon the sidewise shifting of the principallight ray, although the light always passes through the center of thepupil 5 which is also the pupil surface of the object lens 4. In thiscase, for the filtering operation as explained in FIG. 1 to be possiblycarried out by the lens system consisting of the component members 10,11, 12, and 13 as shown in FIG. 1, it is desirable that the diameter ofthe valid light beam in the incident light which has arrived at thepupil surface in the form of a parallel light owing to the relay lensbecomes smaller than the diameter of the pupil surface. Owing to theproperty of the telecentric object lens, the pupil 5, after the light isreflected at the flat portion on the object surface 1, again focusses onthe position of the pupil 5 with an equal magnification. Consequently,the light beam reflected at the flat portion on the object surface againpasses through the position of the pupil 5 with an effective diametersmaller than the pupil diameter. That the principal light ray of thepassing beam unchangeably passes through the center of the pupil 5, inspite of the scanning operation being conducted on the object surface,can be easily understood from the fact that the principal light ray isperpendicularly projected onto the objective with the telecentric objectlens and is reflected to travel back along its original path. The lightwhich has entered into the detection optical system by means of a beamsplitter 7 focusses the pupil 5 on the position of the lightintercepting plate 11 by means of the lens 10. The manner, in which onlythe reflected light component from the flat portion formed at thelocation of the light intercepting plate 11 is removed by a stopper totake out only the light from the edge of the pattern, is exactly thesame as in the case of FIG. 1. Since the incident light beam forms abright spot or a slit on the pupil 5, the filtering operation on theposition of the light intercepting plate 11 which is conjugative withthe pupil is easy. For instance, when the scanning beam is in the formof a spot, the shape of the light beam to be filtered is in a spot.Therefore, on the position of this light intercepting plate 11, theremay be disposed a filter to intercept this spot-shaped light beam suchas, for example, that having a transmitting portion in a ring shape.When the scanning beam is in the form of a slit, the diameter of thelight beam on the pupil surface may sometimes assume a slit-shape. Inthis case, a filter having a slit-shaped light intercepting portion maybe placed at the light intercepting plate 11. In like manner, the shapeof the filter 11 can be arbitrarily varied depending on cases concerned.

FIG. 5 shows another optical layout of an embodiment, wherein thepresent invention is applied to an automatic aligner for an integratedcircuit (IC). For a mask and a wafer to be two-dimensionally aligned bythe use of this automatic aligner, at least two places (or points)should be observed. In this illustration, however, only one of them isshown. In other words, while another optical system for observation anddetection is to be disposed on the left side portion of the drawing, itis omitted, because it is exactly same as the right side portion asillustrated. It is to be noted that the scanner 30 is so constructedthat it may be used commonly. It goes without saying, however, that thenumber of the scanners may be increased depending on the number ofobserving points.

In FIG. 5, a reference numeral 31 designates a light source. Consideringdirectivity and brightness of the beam, as mentioned in the foregoing,use of a laser is convenient, hence the light source 31 in thisillustration is assumed to be the laser. Since the output light of thelaser can be converted to a photo-electric signal with high efficiency,the laser of 1 mW and below will suffice for the purpose of the presentinvention. A reference numeral 32 designates a beam expander whichserves for expanding the laser beam. This component part may, of course,be dispensed with, when the beam diameter is not necessary to beexpanded. A mirror 33 and a lens 34 function to focus the laser beam ona slit (or spot) 35. In this case, the lens 34 should preferably be acylindrical lens, if 35 is in a slit form, while it is sufficient to bean ordinary spherical lens, if 35 is in a spot form. Also, the F-numberof the lens is defined to maintain a relationship with the F-number ofthe relay lens as explained in regard to FIG. 4. Numeral 36 refers to atransmission type scanner which is made of a glass block. Incidentally,it should be understood that the rotational axis of this scanner passesthrough an intersecting point of the three optical axes as shown in thedrawing, and is normal to the drawing sheet, hence three channels ofsignals can be taken out of the block. Consequently, the slit (or spot)is also provided three in number for each optical axis. For eachchannel, there is provided an optical system comprising componentmembers 31 to 34 in front of the slit (or spot) 35. In this illustratedembodiment, however, the optical system of the channel X alone is shown,and the other optical systems are omitted, since they are same inconstruction. It is, of course, possible that light from the lightsource 31 is split by the beam splitter so that the light source to beused may be single.

Light scanned by the scanner passes through an image rotator 37 beforeit reaches a lens 39 (corresponding to the lens 22 in FIG. 3). Assume,for example, that the normal of the three mirror surfaces constitutingthe image rotator of the channel X is in the plane of the drawing sheet.The image rotator 37 (enclosed by dotted lines) is disposed at aposition rotated by 45 degrees with an axis PP' as the rotational axisthereof, whereupon the light which has passed through the channel X isscanned within the plane of the drawing sheet at the points 41 and 42 onthe surface of the object (wherein the numeral 41 refers to a mask andthe numeral 42 refers to a wafer). On the other hand, however, the lightwhich has passed through the channel Y is scanned in the directionnormal to the drawing sheet. That is, as shown in FIG. 6, light from thechannel X and light from the channel Y move each other, as shown in thedrawing, within the sight of the microscope with the consequence thatthe two-dimensional discrepancy between the objects 41 and 42 can bedetected. When the member 35 is in the slit form, the direction of theslit should preferably be set in such a manner, that it may becomeperpendicular to the scanning direction. Incidentally, in FIG. 8, theimage rotator of the channel X is inserted for the purpose of correctingthe light path length. Going back again to FIG. 5, a reference numeral38 designates a beam splitter, and a numeral 39 refers to a relay lens,the construction of both being as shown in FIG. 3. A numeral 40 refersto the telecentric objective lens, 41 indicates a mask, and 42 denotes awafer. The photo-electric detection system consists of members 43, 44,45, and 46, in which 43 refers to an image forming lens of the pupil, 4indicates a stopper, 45 denotes a light converging lens, and 46represents a photo-detector. The entire construction in the neighborhoodof this photo-electric detection system is the same as explained in FIG.3, hence explanations are dispensed with. Incidentally, the beamsplitter 38 may be inserted on the way of the channel X and arrange thesame at the left side so that the left side part may also be scanned asshown by a dotted line in FIG. 5.

FIG. 7 illustrates another layout of the optical system similar to thatshown in FIG. 5. In this embodiment, the optical axis of the object lensis normal to the drawing sheet so as to be able to observe the mask 41and the wafer 42 positioned within a plane parallel with the drawingsheet. With this optical system, two image rotators 37 can be used.Essentially, a single image rotator will, of course, suffice for theintended purpose, but, for correction of the light path length, twoimage rotators are used in this embodiment. The function of the opticalsystem is exactly same as that shown in FIG. 5, hence the explanationthereof is dispensed with.

In FIGS. 5 and 7, the optical system is for observation with the eyes,and the light source for observation to be provided, if need be, are notillustrated. Since their provision can be easily realized either byinserting the beam splitter in one part of the light path, or bychanging the mirror to the beam splitter, they are omitted from theshowing. Further, the constructions shown in FIGS. 5 and 7 make itessential that the image rotator be present. However, by improving thealignment mark on the mask and wafer in some way or other, thediscrepancies in both X and Y directions can be detected at once with aunidirectional scanning. In such case, since the scanning operation canbe performed in a single direction, there is no necessity of insertingthe image rotator, or of introducing beam in two channels with respectto a single sight.

FIG. 8 shows a different embodiment of the optical system of a type, inwhich the scanning optical system causes the scanning beam to deflectwith one point as the center of deflection, for example, an opticallayout wherein the scanning device such as a rotatory polygonal mirrorand a galvano mirror are used. A reference numeral 50 designates a laserbeam, in the path of which a beam expander, or a converging or diverginglens may be inserted, if necessary. Such component member has beenomitted from showing in this illustration for simplification of theexplanations. A numeral 51 refers to a lens for converging the laserbeam, and 52 refers to one surface of a rotary polygonal mirror. Anumeral 53 designates a field lens disposed in the vicinity of aconverging point X of the beam due to the lens 51. The point X movesperpendicularly with respect to the optical axis of the rotatorypolygonal mirror by its rotation. Also, the size of the spot at thepoint X is determined by the F-number of the light beam to be fixed bythe lens 51. A numeral 54 denotes a relay lens, and 4 refers to atelecentric object lens. A numeral 5 represents a pupil, the pupilposition of which corresponds to the object 1. In addition, the systemcomposed of various members from the beam splitter 7 upto and includingthe light detector 13 is the same as that shown in FIG. 3, henceexplanation is omitted. The characteristic in this system is that, whenthe principal light ray of the scanning beam is projected into the relaylens, it is no longer parallel. Accordingly, as has so far beenindicated in FIGS. 1 to 7, the problem is not solved by merely locatingthe pupil of the telecentric object lens on the focal point of the relaylens, but a still different arrangement is required to be taken. Inorder that the beam may not almost move on the pupil surface, even whenthe scanning is conducted on the object surface, attention is given tothe beam reflecting position which is an unmovable point on the rotatorypolygonal mirror. That is, since the reflecting position of lightprojected onto the rotatory polygonal mirror fluctuate to so small adegree that it can be regarded as a substantially unmovable point, thispoint is focussed on the pupil position 5 of the object lens by the useof the field lens 53 and the relay lens 54. In this way, the objectsurface can be scanned, while the beam position on the pupil surface ismaintained unmovable. On the other hand, the surface to be scanned bythe focussing point X of the beam due to the lens 51 is conjugative withthe object surface 1. Consequently, the power of the lens 51 can beprimarily determined from the size of the scanning spot for use inscanning the object surface (i.e., with what micron of the spot, theobject surface is to be scanned), and the diameter of the incident laserbeam. In general, since the diameter of the scanning spot is fairlylarger than the diffraction limit of the object lens, the valid diameterof the incident laser beam on the pupil surface is smaller than thediameter of the pupil surface with the result that the filtering methodas shown in FIG. 3 and others becomes feasible.

As stated in the foregoing, the scanning of the object surface with alight beam and detection of the scanned light beam in a dark sightsystem according to the present invention are more excellent than theconventional methods on all points such as light quantity, improvementin SN ratio, polarity of signal, and other problems, hence the presentinvention has wide varieties of application, not only in the automaticalignment device for IC, but also in other fields such as sizemeasurement, curve tracking, and so forth.

What we claim is:
 1. A device for scanning an object having a flatreflection surface and an inclined surface with a certain inclinationwith respect to the flat reflection surface, comprising:a. means forforming a relatively thin fixed light beam; b. a deflector fordeflecting the light beam from the light forming means to scan acrossthe object; c. a lens system for receiving the deflected light beam fromsaid deflector, including an objective lens facing the object, theoptical axis of which objective lens is perpendicular to the flatreflection surface of said object and having a light source side focalplane, said objective lens being substantially so disposed that saiddeflected light beam continuously passes through the center of saidlight source side focal plane as it is being deflected; d. a lightdetector for detecting light which has been reflected from said objectand passed through said objective lens; and e. optical filter meansinterposed between said objective lens and said light detector, forintercepting light from the flat reflection surface of said object andfor directing the light from the inclined reflection surface to saidlight detector.
 2. The device as claimed in claim 1, wherein said filteris optically located on the focal plane of the objective lens.
 3. Adevice for scanning an object having a flat reflection surface and aninclined surface with a certain inclination with respect to the flatreflection surface, comprising:a. means for forming a relatively thinfixed light beam; b. a deflector for deflecting the light beam from thelight source means to scan across the object; c. a lens system forreceiving the deflected light beam from said deflector, including anobjective lens facing the object, the optical axis of which isperpendicular to the flat reflection surface of said object and having alight source side focal plane of said objective lens; d. image focusingmeans for continuously imaging an image of an original point ofdeflection of said deflected light beam upon the center of said lightsource side focal plane as the light is deflected to scan across theobject; e. a light detector for detecting light which has been reflectedfrom the object and passed through said objective lens; and f. opticalfilter means interposed between said objective lens and said lightdetector, for intercepting light from the flat reflection surface ofsaid object and for directing the light from the inclined reflectorsurface to said light detector.
 4. A device for photoelectricallypositioning a plurality of objects each having an alignment mark whichhas a flat reflection surface and an inclined surface with a certaininclination with respect to the flat reflection surface, comprising:a.means for forming a relatively thin fixed light beam; b. a deflector fordeflecting a light beam from the light source means to scan across theobjects; c. a lens system for receiving the deflected light beam fromsaid deflector, including an objective lens focusing the objects, theoptical axis of which is perpendicular to the flat reflection surface ofsaid objects and having a light source side focal plane of saidobjective lens, said objective lens being substantially so disposed thatsaid deflected light beam continuously passes through the center of saidlight source side focal plane as it is being deflected; d. a lightdetector for detecting light reflected from said objects and passedthrough said objective lens; e. optical filter means interposed betweensaid objective lens and said light detector, for intercepting light fromthe flat reflection surfaces of said objects and for directing the lightfrom the inclined reflection surfaces to said light detector; and f.drive means for causing the relative position of said plural objects tochange in accordance with signals obtained by said light detector.